Styracosaurus (meaning "spiked lizard")[1] was a genus of herbivorousceratopsiandinosaurs from the CretaceousPeriod (Campanianstage), about 76.5 to 75.0 million years ago. It had four to six long horns extending from its neckfrill, a smaller horn on each of its cheeks, and a single horn protruding from its nose, which may have been up to 60 centimeters (2 ft) long and 15 centimeters (6 in) wide. The function or functions of the horns and frills have been debated for many years.

Styracosaurus was a relatively large dinosaur, reaching lengths of 5.5 meters (18 ft) and weighing nearly 3 tons. It stood about 1.8 meters (6 ft) tall. Styracosaurus possessed four short legs and a bulky body. Its tail was rather short. The skull had a beak and shearing cheek teeth arranged in continuous dental batteries, suggesting that the animal sliced up plants. Like other ceratopsians, this dinosaur may have been a herd animal, traveling in large groups, as suggested by bonebeds.

Named by Lawrence Lambe in 1913, Styracosaurus is a member of the Centrosaurinae. Two species, S. albertensis and S. ovatus are currently assigned to Styracosaurus. Other species assigned to the genus have since been reassigned elsewhere.

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Individuals of the Styracosaurusgenus were approximately 5.5 metres (18 ft) long as adults and weighed around 2.7 tons.[2] The skull was massive, with a large nostril, a tall straight nose horn, and a parietosquamosal frill (a neck frill) crowned with at least four large spikes. Each of the four longest frill spines was comparable in length to the nose horn, at 50 to 55 centimetres long (19.7 to 21.7 in).[3] The nasal horn is estimated at 57 centimeters long (19.7 in) in the type specimen,[4] but the horn is only partially complete. Based on other nasal horn cores from Styracosaurus and Centrosaurus, this horn may have come to a rounded point at around half of that length.[5]

Aside from the large nasal horn and four long frill spikes, the cranial ornamentation was variable. Some individuals had small hook-like projections and knobs at the posterior margin of the frill, similar to but smaller than those in Centrosaurus. Others had less prominent tabs. Some, like the type individual, had a third pair of long frill spikes. Others had much smaller projections, and small points are found on the side margins of some but not all specimens. Modest pyramid-shaped brow horns were present in subadults, but were replaced by pits in adults.[5] Like most ceratopsids, Styracosaurus had large fenestrae (skull openings) in its frill. The front of the mouth had a toothless beak.

The bulky body of Styracosaurus resembled that of a rhinoceros. It had powerful shoulders which may have been useful in intraspecies combat. Styracosaurus had a relatively short tail. Each toe bore a hooflike ungual which was sheathed in horn.[2]

Various limb positions have been proposed for Styracosaurus and ceratopsids in general, including forelegs which were held underneath the body, or, alternatively, held in a sprawling position. The most recent work has put forward an intermediate crouched position as most likely.

Styracosaurus is a member of the Centrosaurinae, a subfamily of large North American horned dinosaurs characterized by their "prominent nasal horns, subordinate brow horns, short squamosals in a short frill, a tall, deep face relative to the chasmosaurines, and a projection into the rear of the nasal fenestra."[7] Other members of the clade include Centrosaurus (from which the group takes its name),[8][9] Pachyrhinosaurus,[8][10] Avaceratops,[8] Einiosaurus,[10][11] Albertaceratops,[11] Achelousaurus,[10] Brachyceratops,[12] and Monoclonius,[8] although these last two are dubious. Because of the variation between species and even individual specimens of centrosaurines, there has been much debate over which genera and species are valid, particularly whether Centrosaurus and/or Monoclonius are valid genera, undiagnosable, or possibly members of the opposite sex. In 1996, Peter Dodson found enough variation between Centrosaurus, Styracosaurus, and Monoclonius to warrant separate genera, and that Styracosaurus resembled Centrosaurus more closely than either resembled Monoclonius. Dodson also believed one species of Monoclonius, M. nasicornis, may actually have been a female Styracosaurus.[13] However, most other researchers have not accepted Monoclonius nasicornis as a female Styracosaurus, instead regarding it as a synonym of Centrosaurus apertus.[5][14] While sexual dimorphism has been proposed for an earlier ceratopsian, Protoceratops,[15] there is no firm evidence for sexual dimorphism in any ceratopsid.[16][17][18]

Goodwin and colleagues proposed in 1992 that Styracosaurus was part of the lineage leading to Einiosaurus, Achelousaurus and Pachyrhinosaurus. This was based on a series of fossil skulls from the Two Medicine Formation of Montana.[19] The position of Styracosaurus in this lineage is now equivocal, as the remains that were thought to represent Styracosaurus have been transferred to the genus Rubeosaurus.[20]